feat(notes): banner when viewing an older shared version

Cache the note path on each snapshot so an old shared sha can find its
current version, and show a calm "older shared version — View latest"
banner. The exact snapshot is shown as-is; the banner only offers a jump
to the latest, never swapping content.

src/modules/note/snapshotStatus.ts

@@ -0,0 +1,25 @@
+/**
+ * Return the note's current (latest) sha when `viewedSha` is NOT it — i.e. you
+ * are viewing an older version — otherwise null. "Older" is not inferred from
+ * the sha (a content hash has no order); it means "not the current sha for this
+ * path" per the repo file list.
+ *
+ * The caller must supply `notePath`. For a current sha it comes from the file
+ * list; for an older sha it comes from the cached snapshot's stored `path`
+ * (notes viewed while current carry it). When the path can't be resolved
+ * (foreign / evicted / pre-upgrade snapshot) this returns null and no banner is
+ * shown — graceful, never a false claim. Content is never swapped.
+ */
+export const latestShaIfOlder = (
+  viewedSha: string,
+  notePath: string | undefined,
+  files: ReadonlyArray<{ path?: string; sha?: string }>
+): string | null => {
+  if (!notePath) {
+    return null
+  }
+
+  const currentSha = files.find((file) => file.path === notePath)?.sha
+
+  return currentSha && currentSha !== viewedSha ? currentSha : null
+}